Airless spraying of high-viscosity as well as low-viscosity materials has found increasing acceptance particularly since no or only insignificant spray-mist problems occur, since there is no paint bounce-back from corners and recesses, and since a clean environment is maintained.
In addition, highly-viscous materials can be used without difficulties and in higher film thicknesses. A complete filling of pores on any substrate is obtained resulting in improved adhesion and, thus, in an application of better quality.
The method of operation for the so-called airless spray method consists in a high-pressure pump which draws the spray material from any type of container by suction and delivers it to a spray gun by means of a special hose under high hydraulic pressure. Through a specially shaped nozzle the material is atomized without air into minute droplets and applied to the workpiece at high velocity. This makes it possible to cover edges and corners of complicated workpieces as uniformly as smooth surfaces.
One problem in the airless-spay method consists in the requirement that a high holding pressure must also be maintained during idle times in order to make it possible to continue spraying immediately and with the same quality. Piston pumps and in particular differential piston pumps serve as high-pressure pumps. As a drive motor, either an inline or rotary motor can be used. In the rotary motor, the rotational motion of the motor is converted into a reciprocating motion for the movement of the piston. This is accomplished by mounting to the drive-motor shaft an eccentric disk which is connected to the piston rod of the piston pump by means of a connecting rod. In the top dead center and bottom dead center of the piston rod travel no material is being moved which results in an uneven spray pattern. These fluctuations in the spray pattern increase with the amount of material exiting the spray nozzle. It is not possible to prevent this fluctuating spray pattern with presently known means.
A compressed-air driven, inline motor serves primarily as a driver for the piston of the piston pump since this motor allows for infinitely variable material pressure whereby the amount of material regulates itself. The compressed-air motor only operates with an open spray gun; it stops, however, after the spray gun is closed and resumes running as soon as the gun is opened again. Its running speed is automatically governed by the flow-rate at the spray nozzle.
Problems occur with the use of other drive motors since the spray pressure in the spray gun must be maintained during idle time. With the use of electric motors, one, therefore, installed a spill valve which recirculates the unused, excess paint. The disadvantage of this system consists on the one hand in an increase in pump wear and, on the other hand, in a continuous circulation of paint material which is not permissible with filler paints, for example. Furthermore, the insertion of a hydraulic circuit between motor and pump is known. This has the disadvantage that a very high pressure drop occurs at the dead centers due to the inertia of the hydraulic circuit. The pressure drop which amounts to approximately 5 to 10% in a mechanical circuit assumes considerably higher values in a hydraulic circuit. This restricts the use of such units to applications in which demands for quality are low.
In addition, DC motors have been used recently as drive motors which are controlled and which make it possible, therefore, to stop and restart under load. However, the expenditure for control technology is considerable and makes this drive expensive; furthermore, this drive is also susceptible to malfunctions due to the frequently very rough use in shipyards, construction sites, etc.
Basic to the invention is the provision of a device for the drive of pumps with constant holding and preset operating pressure which makes it possible to use a commercially available electric motor, gasoline engine, or also air motor, which can continue running even with a closed spray gun; which makes it possible to make infinitely variable adjustments to the operating pressure between a value maximally limited by the pump and zero; and which, in addition, when used in connection with piston pumps considerably decreases the pressure fluctuations in the spray stream as compared to presently known pump configurations.
A device according to the invention for the drive of pumps has, therefore, a differential gear, whereby the drive motor is connected to the drive shaft. E.g., an electric motor, a combustion engine, or also an air motor can be used as a drive motor. This drive motor runs continuously with a defined rpm. The pump is connected to one of the two output shafts of the differential gear; to the other a brake unit is connected which preferably consists of a hydraulic pump with a hydraulic circuit in which an adjustable throttle valve is inserted. The pressure of the pump is regulated by means of the brake unit. When the spray gun is closed and thus no material is moved, the pressure in the pump increases to such a level that the output shaft of the differential gear connected to the pump is retarded to zero. Because of the differential gear, only the output shaft leading to the brake unit continues to be driven. Thus, the motor can continue to run with the spray gun closed without being damaged while the previously selected operating pressure is being maintained at the pump.
A further advantage of a device according to the invention for the drive of pumps consists in the fact that based on the employed differential gear with the use of a piston pump, the pressure drop at the dear centers, i.e., the points of reversal of the piston, is no longer as large as with known pump drives because, due to the use of the differential gear, the rpm of the drive shaft of the piston pump increases quite considerably, especially in the dead centers, as compared to the speeds at a corresponding angle of 90.degree. because the resistance has been considerably reduced due to the small load in these ranges. The result is that the time interval of pressure-drop as compared to the intervals with normal operating pressure has decreased to such a considerable percentage that this pressure drop can hardly be noticed in the spray pattern and this even with high pump loads.